The present invention relates generally to stopper rods for controlling the flow of molten ferrous metals and more particularly, to one-piece stopper rods which incorporate means for introducing an inert gas, such as argon, to the melt during casting operations.
In the art of continuous casting, it is well known to use one-piece refractory stopper rods for the control of molten metal flowing from a tundish to a water cooled mold. The stopper rod is moved up and down by the use of a rigging on the outside of the tundish to control the molten metal flow. While the principal is quite simple the working environment is very harsh. A refractory stopper rod must be able to withstand hours submerged in molten steel. It must also be capable of enduring the harsh thermal shock encountered on the start-up of casting.
In recent years, the one-piece stopper rod has been used to introduce an inert gas, usually argon gas, into the molten metal. The argon gas serves several purposes in the continuous casting process. First, the non-metallic inclusions in the molten metal are floated out as the gas bubbles upwardly through the metal in the tundish. The rounded portion at the nose of the stopper rod is in contact with a submerged entry nozzle which protects the stream as it exists the tundish and flows into the continuous casting mold. A problem frequently encountered in the continuous casting of steel is clogging of this submerged entry nozzle due to aluminum oxide present in the molten steel. Argon injection through the stopper rod above the nozzle also minimizes this problem.
It is often very difficult to obtain a gas tight seal at the top of the stopper rod where it connects to the rigging on the tundish. The gas tight seal is important due to the fact that the flow of the steel from the tundish to the casting mold creates a vacuum. This vacuum can draw air through the stopper rod and into contact with the molten metal, causing oxidation and a subsequent reduction in the quality of the metal being cast. Injection of argon through the bore of the stopper rod eliminates this potential problem by creating a positive pressure inside the stopper.
In present day steel making, the injection of argon through a one-piece stopper rod has become the industry standard for the continuous casting of steel. In order to meet the industry requirements, a number of stopper rod designs are presently utilized to inject argon into a tundish and continuous casting nozzle. While these designs generally meet the requirements of steel makers, they often have significant limitations. An earlier attempt to introduce inert gas to a stopper rod consists of a one-piece stopper rod with a hole either pressed or drilled through the end of the rod. This solution has several major problems. First, if for any reason, the argon flow is interrupted, the rod bore fills up with steel due to the ferrostatic pressure in the tundish, making it necessary to terminate the cast. The stopper rod hole, which is generally about 2 to 3 mm in diameter, also produces very large argon bubbles. Large bubbles are not as effective as small bubbles in cleaning the steel by raising inclusions. In addition, with a hole completely through the rod, it is difficult to maintain gas pressure within the rod, thus making gas flow control difficult.
In order to overcome the above problems, a further design has evolved. In this prior approach, a prefabricated porous refractory plug is cemented into a bore pressed through the nose of the stopper rod. The plug is generally a high alumina ceramic bonded composition with the permeability controlled by a technique known as gap-grain sizing. This technique controls the pore size by controlling the grain sizing of the raw materials used to fabircate a ceramic body. This approach overcomes several of the problems existing in the above described simple hole type design. First, the porous plug gives very fine bubble dispersion, and, as such, is effective in cleaning the steel. The porous plug also allows the creation of back pressure in the rod bore so that gas flow can be more easily controlled. The problems encountered with this type of design concern the loss of the porous plug during the casting whenever the cement joint fails. If too high a gas pressure is exerted on the plug, it can be blown completely out of the rod. The loss of the porous plug is catastrophic, again causing the rod bore to fill up with steel and halting the casting run.
A still further design of the stopper rod heretofore proposed comprises a composite of the aforementioned porous plug and the small diameter nose hole types. In this prior approach, a preformed porous plug is co-pressed into the bore of the stopper rod upstream of a smaller diameter nose hole during the manufacturing process. This design offers an improved degree of safety due to the fact that the porous plug cannot be lost in use. The porous plug also guarantees the maintenance of a positive pressure upstream in the stopper rod bore. The disadvantage of this type of design is that the desired effect of the fine bubbles of inert gas is lost due to the presence of the hole between the porous plug and the stopper nose. In addition, this design is quite difficult to manufacture. Pressing the plug in the stopper rod at the high pressure used in the manufacturing of the stopper rod often destroys the integrity of the porous plug. This makes it difficult to manufacture a reproducible product. Any material that accidentally comes between the plug and the channel during the assembly of the tooling prior to the isostatic pressing of the rod will block the plug, consequently blocking the flow of gas. No solution has been found that completely meets the requirements of steel makers by solving the difficulties previously mentioned.
The present invention solves the problems heretofore encountered in the prior art by providing a one-piece stopper rod with an integral porous nose which delivers a fine dispersion of inert gas bubbles to the molten metal with no danger of porous plug blowout. The invention also prevents the backflow of molten metal into the bore of the stopper rod in the event inert gas flow is interrupted. The invention further provides a one-piece refractory stopper rod possessing high resistance to thermal shock and steel erosion while retaining the benefits of a gas permeable nose portion. The invention still further provides a one-piece stopper rod with a porous nose which permits maintenance of sufficient gas pressure within the rod to achieve uniform gas flow therethrough.